Vacuum flask



FIG. 1 is a top perspective view of my new design for a vacuum flask;

FIG. 2 is a bottom perspective view thereof;

FIG. 3 is a top view thereof;

FIG. 4 is a bottom view thereof;

FIG. 5 is a side elevation view thereof; and,

FIG. 6 is a front elevation view thereof. 

CLAIM The ornamental design for a vacuum flask, as shown and described. 